1. Field of the Invention
The present invention relates generally to tape drive performance enhancement and data reliability improvement and more specifically to a method and apparatus for accurately writing data to tape media having very high storage capacities.
2. Description of the Prior Art
For many years tape drive subsystems (hereinafter referred to as drive units) associated with data processing systems have used a single conceptual model for recording data on tape. While the specifics of the implementations have varied, data have historically been written to tape in units known as blocks. Blocks of data have been separated by a space on the tape medium known as an interblock gap (IBG). IBGs provide the ability to format recorded signals on the tape medium. Each IBG can be either an erased portion of the tape or a systematic pattern of data written to the tape easily recognizable by the drive unit as an IBG.
The required length of an IBG to effect proper tape drive subsystem performance depends on a number of factors. Such factors include the performance level of the transport motor (i.e. its ability to accelerate the tape) and the tape operating speed. IBG length can further be dictated by industry, national and international standards. High tape speeds and lower-acceleration transports require longer IBG lengths. On the other hand, shorter interblock gaps require lower tape operating speeds and/or higher performance transport mechanisms.
Certain types of applications require high speed tape movement over the read and write heads of the tape drive. Such applications include archiving, disaster recovery and other operations involving bulk data transfers. These types of applications are referred to in the industry as streaming applications. These applications require high tape speed movement to achieve a correspondingly high data transfer rate so that bulk data transfers can occur within a reasonable amount of time in order to meet customer requirements.
In these applications, a new read or write instruction is typically received by the drive unit either before or during the time when the IBG on the tape is passing the read and write heads. This should provide sufficient time for the new block of data to be processed within the drive unit for transmission to or read from the tape medium. In the case of a write append, if tape speed is too fast or the IBG length too short, a "backhitch" will be necessary because the new write command will not be ready for transmission to tape by the time that the write head reaches the end of the IBG. During a backhitch, the tape is brought to a stop downstream of the IBG, then driven in a reverse direction and caused to come to rest at a point upstream of the IBG. The tape then remains stationary until the write instruction has been fully processed by the drive unit, whereupon the tape is accelerated through the IBG such that it is at operational speed by the time the head is positioned one-half way through the IBG at the point when the write drivers are activated.
As mentioned above, these high speed applications, in order to function properly, have heretofore required either a longer IBG length or a higher performance transport system able to very quickly accelerate. Neither solution has been ideal since the former significantly reduces data storage capacity and the latter requires inclusion of an expensive component.
It can be seen that by increasing IBG length the resulting data capacity of a particular storage medium is decreased from what it would have been with shorter IBG lengths. While block data recording densities have increased from about 100 bytes per inch in the IBM 726 tape drive (announced in 1952) to about 77,000 bytes per inch in the IBM 3490E Tape Drive (announced in 1991), there has not been a corresponding increase in effective data recording densities due to the presence of IBGs.
While high performance tape transport mechanisms have correspondingly increased in performance over the years, they are considerably more expensive than their lowerperformance counterparts. Moreover, tape wear is significantly increased with higher tape accelerations as is the possibility of tape damage. This is especially true as a result of the thinner tapes being used in the drives of today and those planned for the future.
One particular failure mechanism present with current tape drives is a result of Inter Layer Slip (ILS). The reduced tape thickness of recent and future tape media products increases the tendency of the tape medium to "slip" during winding or unwinding of the tape to or from a tape reel. The reel-to-reel tape drives of today operate under the assumption that the tape wound around each of the reels is at the correct tension and that there are no loose wraps between layers of the tape that might cause the reels to falter during the winding and reeling process.
In reality, various environmental factors can cause the tape wound on a reel to expand and contract thereby changing the tension in the reel and causing loose wraps. When the drive attempts to rotate these loose wraps on the reel hub, inter layer slip results. After initially loading a tape cartridge, ILS is most prevalent during the stop and start motions of a tape drive associated with a "write append" operation.
The write append operation is one of the more common operations performed by a tape drive. When new data are to be written to tape they can either be written after the end of all current data residing on the tape or they can be written over existing data on the tape. In the first case, data are written following the IBG written after the last data block and over an EOD (End of Data) mark. In the second case, data are written over existing data within the same area that defined a block for the overwritten data.
During a write append operation, the drive must first locate the IBG just before the intended append point by reading existing blocks using the read head. The write head physically leads the read head with respect to the tape movement. This is because the tape drive performs a read after write operation to verify the write. This must be done without rewinding the tape. Because the physical spacing between the read head and the write head is greater than the nominal length of an IBG, the write head has heretofore been positioned solely through the use of mechanically measured tachometer for a write append operation.
The interlayer slip that can occur during this stop/start/stop/start operation prevents the drive unit from accurately locating the IBG. This, in turn, results in a requirement that the IBG be of a larger size to ensure accuracy, thus degrading tape capacity.
If there is an undetected positioning problem during the write append it is possible that the previous block on the tape may be overwritten. Moreover, this error will not be detected until the read after write when it is too late to correct the problem. As can be imagined, this loss of customer data is unacceptable.
The use of a tach count alone to locate the IBG during a write append has an additional limitation. The drive starts writing a portion of the IBG and then customer data as soon as the appropriate tach count is observed. This does not guarantee that drive speed variations are contained within the allowable range to ensure a proper write. As a result of this speed variation, the beginning of the block is written at a different linear density. The variations may not be noticeable during the read verify after the write since the tape is moving past the read and write heads at the same linear velocity. This is especially true because of the small spacing between the read and write head elements in modern tape drives.
Thus, the data block will pass the read while write thresholds. Later, when the data are being read, the initial portion of the block becomes difficult to acquire in the logical forward direction since the data had been written at an improper linear density at the beginning of the block. A read temporary error is generated as a result. The data can then often be acquired only in read backward mode after the error recovery process (ERP) is initiated. The increase in read temporary errors and ERPs affects the drive unit reliability and overall processing speed.